The present invention relates to an optoelectronic transmitter having an improved control circuit for driving a semiconductor laser. The transmitter disclosed herein may also be included with an optoelectronic transceiver which incorporates both an optical transmitter and an optical receiver for providing bidirectional data communication. A first embodiment provides Automatic Power Control and laser slope compensation in a small footprint transceiver package. A second embodiment also provides Automatic Power Control and laser slope compensation, but within a larger transceiver package. The larger package of the second embodiment allows for additional circuitry for providing laser fault latching. In both embodiments, the control circuit is configured such that all variable components associated with normalizing the output characteristics of the semiconductor laser may be mounted on a separate carrier printed circuit board apart from the remaining components, requiring only a small number of connection points between the main transceiver printed circuit board and the carrier printed circuit board. The invention further includes an improved technique for mounting a Transmitting Optical Sub-Assembly to the carrier printed circuit board, and attaching the carrier printed circuit board to the main transceiver printed circuit board at a 90.degree. angle.
Optoelectronic transceivers are well known in the art. For example, a 1.times.9 package is a common small footprint transceiver housed in a compact, standardized modular package. Functionally, small footprint optoelectronic transceivers provide bidirectional data communication between an electronic host device and a pair of optical fibers. Binary voltage signals generated within the host device are input to the transceiver where they are converted into optical signals to be transmitted over a first optical fiber. On the receiver side, binary optical signals are received over a second optical fiber and converted to voltage signals which can be read by the host device. Physically the small footprint package generally comprises a horizontal printed circuit board with the electronic components necessary to drive the optical transmitter and receiver mounted thereon. Optical sub-assemblies including a Transmit Optical Sub-Assembly (TOSA) and a Receive Optical Sub-Assembly (ROSA) are mounted beside one another along one edge of the printed circuit board. The TOSA and ROSA provide the optical interface between the optical fibers and the small footprint transceiver package. The optical subassemblies are mounted such that the optical axis of both optical sub-assemblies are parallel with the plane of the main transceiver printed circuit board, pointing away from the transceiver package. An electrical connector comprising a single row of nine contact pins is mounted along the opposite edge of the printed circuit board, providing the electrical interface between the transceiver package and the host device.
A problem with prior art small footprint transceiver packages has been in mounting the TOSA and ROSA to the main transceiver printed circuit board. The TOSA and ROSA are generally cylindrical in shape, and in order for the optical axes of the subassemblies to be parallel with the printed circuit, the base of the subassemblies must be mounted perpendicular to the surface of the printed circuit board. The optical sub-assemblies must be somehow held firmly in place relative to the printed circuit board, and the electrical leads extending from the sub-assemblies must be securely and reliably bonded to the driver and receiver circuits formed on the main transceiver printed circuit board. For example, in U.S. Pat. No. 5,528,408, issued to McGinley, et al. and assigned to Methode Electronics, Inc., the TOSA, ROSA and main transceiver printed circuit board are all housed within an outer housing. The cylindrical bodies of the TOSA and ROSA are modified to mate with mounting structures formed within the housing. Likewise, the main transceiver printed circuit board is also held in place by additional mounting structures formed within the housing. The mounting structures are configured such that the printed circuit board is held in fixed horizontal relation below the TOSA and ROSA, with the optical axes of the TOSA and ROSA extending parallel to the surface of the main transceiver printed circuit board. Flexible leads extend out the back of the optical sub-assemblies and are formed and soldered to contacts on the printed circuit board. The rigid outer housing maintains the spatial relationship between the optical sub-assemblies and the printed circuit board, thereby alleviating stress on the electrical leads, and preventing the leads from breaking off.
Another solution to the optical sub-assembly mounting problem has been to mount the TOSA and ROSA on a separate printed circuit board connected to the main printed circuit board via flexible circuitry. The TOSA and ROSA are mounted to the second printed circuit board such that the optical axes of the optical sub-assemblies extend perpendicular to the surface of the second board. When the internal electronics are packaged within an outer housing, the flex circuitry is bent 90.degree. so that the second printed circuit board sits perpendicular to the main printed circuit board, and the optical axes of the TOSA and ROSA extend parallel to the surface of the main board.
While both of the examples given above effectively solve the optical sub-assembly mounting problem, both solutions are expensive to implement and add cost to the final product. Among the improvements of the present invention is to provide an improved attachment mechanism for connecting the TOSA and ROSA to the main transceiver printed circuit board.
In addition to the physical mounting provisions of the TOSA and ROSA, the present invention also includes improvements to the control circuit for driving a semiconductor laser as the active optical element within the TOSA. In the past, most optoelectronic transceivers have employed LEDs as the active optical element within the TOSA. More recently, however, LEDs have been replaced by semiconductor lasers. The 5,528,408 Patent, for example, describes a Small Footprint Optoelectronic Transceiver with Laser. The 5,528,408 patent generally describes a small footprint package employing a semiconductor laser such as a Vertical Cavity Surface Emitting Laser (VCSEL) as the active optical element within the TOSA. Employing a semiconductor laser as the active optical element provides a number of advantages over LEDs, including improved coupling efficiency and higher data rates. On the other hand, employing a semiconductor laser gives rise to biasing problems not encountered with LEDs.
One of the primary difficulties with semiconductor lasers is that each individual laser has its own unique set of output characteristics. For example, FIG. 1 shows typical output power versus input current curves, or P-I curves, for three individual semiconductor lasers A, B and C. In FIG. 1, the X-axis represents the drive current input to the semiconductor laser, and the Y-axis represents the corresponding optical output power delivered by the laser. As can be seen, a uniform DC input current I.sub.Q supplied to each of the individual semiconductor lasers A, B and C results in a different amount of optical output power, P.sub.QA, P.sub.QB, and P.sub.QC, being delivered by each of the semiconductor lasers. Furthermore, since the linear operating range for each semiconductor laser has a different slope, a given change in the input current .+-..DELTA.I will cause a greater or lesser change in the output power .+-..DELTA.P for each semiconductor laser. These variations in the slope efficiency of the various semiconductor lasers can also be seen in FIG. 1. The uniform DC operating current, or quiescent current, I.sub.Q is applied to each of the three semiconductor lasers A, B, and C, and an identical alternating current signal I.sub.SIG is superimposed thereon. I.sub.SIG causes a periodic change in the input current +.DELTA.I above and below the quiescent current I.sub.Q. The magnitude of the .DELTA.I applied to each semiconductor laser in FIG. 1 is identical between the three semiconductor lasers A, B, and C. On the output side, however, where the resultant changes in the output power .+-..DELTA.P.sub.A, .+-..DELTA.P.sub.B and .+-..DELTA.P.sub.C generated due to the changes in the input current, vary from one laser to the other. As is clear from the drawing, .DELTA.P.sub.A is greater than .DELTA.P.sub.B, and .DELTA.P.sub.B is greater than .DELTA.P.sub.C. These variations in the output characteristics of individual semiconductor lasers raise a significant barrier to designing a standard, reliable optoelectronic transceiver suitable for mass production.
Ideally, each optoelectronic transceiver of a particular design will have similar output characteristics. The optical output of the transceiver is to represent a binary data signal comprising a serial string of 1's and 0's. A binary 1 is transmitted when the optical output of the transmitter exceeds a certain power threshold, and a binary zero is transmitted when the optical output power of the transmitter falls below a certain power threshold. Maximizing the difference in transmitted power levels between 1's and 0's improves the reliability of the transceiver and improves the sensitivity of the receiver at the opposite end of the data link. Thus, in a transceiver design incorporating a semiconductor laser as the active optical element, the transceiver should include provisions for optimizing the output characteristics of the semiconductor laser. Furthermore, these output characteristics should be the same from one transceiver to another. Therefore, the optimizing circuitry should also normalize the output characteristics of the transceiver to a well-defined standard.
In generating the optical output signal, the transmitter driver circuit receives a binary voltage signal from the host device. The driver circuit converts the input voltage signal to a current signal which drives the semiconductor laser. A signal voltage corresponding to a binary 1 must be converted to a current supplied to the semiconductor laser sufficient to cause the semiconductor laser to radiate an optical output signal having an output power level above the power threshold corresponding to the transmission of a binary 1. Similarly, a signal voltage corresponding to a binary 0 must be converted to a current level supplied to the semiconductor laser which will cause the semiconductor laser to radiate an optical output signal having an output power level below the power threshold corresponding to the transmission of a binary 0. However, due to variations in the P-I characteristics from one semiconductor laser to another, the current levels necessary to produce the desired output power levels will vary depending on the individual characteristics of each individual semiconductor laser.
In general, variations in the P-I characteristics of individual semiconductor lasers can be compensated for by employing biasing and compensating resistors which shape the input current driving the semiconductor laser. By properly sizing the biasing and compensating resistors, the input current delivered to the laser can be manipulated so that the optical power emitted by the laser behaves in a predictable and beneficial manner. There are two components to the process of properly biasing the semiconductor laser. The first component, automatic power control (APC), involves establishing the average DC input current, or quiescent operating current I.sub.Q. I.sub.Q establishes the average output power, or quiescent operating power P.sub.Q that will be radiated by the semiconductor laser. The second component, laser slope compensation, involves determining the amount of change in the input current, .+-..DELTA.I, necessary to effect the desired change in the output power .+-..DELTA.P to distinguish between 1's and 0's transmitted by the transceiver.
APC and laser slope compensation are best described in conjunction with the P-I curves shown in FIG. 2. FIG. 2 shows the identical P-I curves for semiconductor lasers A, B, and C as shown as in FIG. 1. However, rather than applying the same input current to each device, diverse input currents I.sub.A, I.sub.B, and I.sub.C are applied to each semiconductor laser A, B, and C respectively, such that each laser emits approximately the same output power P.sub.Q. The optical power signal transmitted by the transceiver alternates above and below the quiescent output power P.sub.Q. Binary 1's are represented as P.sub.Q +.DELTA.P, and binary 0's are represented as P.sub.Q -.DELTA.P. The differences in power levels between 1's and 0's comprises the extinction ratio of the transmitter. A greater extinction ratio, meaning a greater difference in the output power levels between transmitted 1's and 0's, results in improved receiver sensitivity at the opposite end of the data link. Therefore, it is desirable to maximize .DELTA.P in order to maximize the extinction ratio. To maximize .DELTA.P it is best to establish a quiescent operating power P.sub.Q near the midpoint of the operating range of the semiconductor laser. Once P.sub.Q has been established, the extinction ratio can be maximized by setting I.sub.Q +.DELTA.P as near the maximum output level of the semiconductor laser as possible, and setting I.sub.Q -.DELTA.P as near the lasing power threshold of the semiconductor laser as possible.
As is clear from FIG. 2, the quiescent input current I.sub.Q, necessary to achieve the same, or nearly the same, quiescent output power P.sub.Q, will vary significantly depending on whether semiconductor laser A, B or C is employed. Automatic power control establishes the quiescent input current I.sub.Q so that the desired average output power P.sub.Q is radiated by the particular semiconductor laser employed in the transceiver. Thus, for the three semiconductor lasers A, B and C depicted in FIG. 2, the quiescent operating currents I.sub.QA, I.sub.QB, and I.sub.QC will each deliver an output power of approximately P.sub.Q from semiconductor lasers A, B, and C respectively. Determining the proper quiescent current I.sub.Q for a particular semiconductor laser involves individually testing the semiconductor laser and varying the input current supplied thereto until the desired output power is achieved. Once the quiescent current I.sub.Q has been determined, a bias circuit can be derived which supplies an average DC input current to the semiconductor laser equal to I.sub.Q.
Once the quiescent current has been established, it is necessary to compensate for the varying slopes of the P-I curves for the various semiconductor lasers. Because the slope of each P-I curve is different, the magnitude of change in the input current .DELTA.I necessary to cause a desired change in the output power .DELTA.P will vary from one semiconductor laser to another. Referring again to the P-I curves of FIG. 2, an alternating current signal I.sub.SIG is superimposed on each of the quiescent currents I.sub.QA, I.sub.QB, and I.sub.QC supplied to each of the semiconductor lasers A, B, and C respectively. The peak magnitudes of the alternating current signals are represented by the quantities .DELTA.I.sub.A, .DELTA.I.sub.B, and .DELTA.I.sub.C. Clearly, .DELTA.I.sub.C is greater than .DELTA.I.sub.B and .DELTA.I.sub.B is greater than .DELTA.I.sub.A, yet for each semiconductor laser the corresponding change in the output power .DELTA.P is approximately the same for each device. By controlling the peak magnitude of the input current signal I.sub.SIG, the quantities .DELTA.I.sub.A, .DELTA.I.sub.B, and .DELTA.I.sub.C can be tailored to the specific slope characteristics of a particular semiconductor laser such that the peak change in the output power .DELTA.P can be set at or near the operating limits of the particular semiconductor laser. Maximizing .DELTA.P maximizes the extinction ratio, thereby optimizing the performance of the transceiver.
Since the binary signals to be transmitted by the transceiver are optical representations of a voltage signal received from the host device, the driver circuit must convert the received voltage signal into the alternating current signal I.sub.SIG and superimpose I.sub.SIG onto the DC quiescent current I.sub.Q. In tailoring the AC current signal to a particular semiconductor laser, the laser slope compensation circuit establishes the peak magnitude of I.sub.SIG resulting from changes in the input voltage signal. The slope compensated AC drive current I.sub.SIG will vary between I.sub.Q +.DELTA.I and I.sub.Q -.DELTA.I, where I.sub.Q and .DELTA.I have been calculated to provide the maximum extinction ratio possible for the particular laser employed. Thus, for example, if semiconductor laser B of FIG. 2 is employed, the slope compensation circuit must supply I.sub.SIGB having peak values of I.sub.QB +.DELTA.I.sub.B and I.sub.QB -.DELTA.I.sub.B, where the quantity .+-..DELTA.I.sub.B has been calculated to maximize the change in output power .DELTA.P of semiconductor laser B.
APC and laser slope compensation are generally accomplished through biasing and compensating resistors included within the driver circuit. When the required quiescent current I.sub.Q and the proper magnitude of the slope compensated current signal I.sub.SIG have been determined, the biasing and compensating resistors can be sized so that the driver circuit supplies the proper current signal to the semiconductor laser. APC and laser slope compensation help to normalize the output characteristics of the optoelectronic transceiver so that individual transceivers may be used interchangeably without having a noticeable effect on the overall data communication system.
A problem with implementing APC and laser slope compensation, however, is that they complicate the manufacturing process and add cost to the final transceiver product. Individually testing the output characteristics of each semiconductor laser is time consuming and expensive. Individually calculating the size of each biasing and compensating resistor to optimize the output characteristics of each device is expensive as well. What is more, having individualized components prevents the main transceiver printed circuit boards from being manufactured in a completely automated fashion. Instead, individual resistors must be sized and soldered in place by hand, once again adding time and cost to the manufacturing process. Therefore, a less expensive method is desired for providing APC and laser slope compensation in optoelectronic transceiver modules where semiconductor lasers are employed as the active optical element.
In addition to APC and laser slope compensation, in some applications it is also advantageous to monitor the output power emitted by the semiconductor laser to ensure that the laser is operating within safe limits. Because the optical energy emitted by a semiconductor laser has the potential to be harmful to the eyes if transmitted with sufficient power, it is prudent to provide a mechanism for disabling the laser when the output of the laser exceeds safe operating levels. Such a mechanism should prevent the drive current from reaching the laser, and should provide a signal to the host device indicating that a laser fault has occurred.
In general, the present invention provides an improved optoelectronic transmitter having an improved mechanism for attaching a TOSA at a right angle to the edge of a main transmitter printed circuit board. The present invention further provides an optoelectronic transmitter package employing a semiconductor laser as the active optical element and wherein an improved control circuit includes components which can be divided between at least two printed circuit boards, requiring only a minimal number of electrical connections therebetween. The improved control circuit must be configured such that all of the variable components associated with APC and laser slope compensation can be mounted on a separate carrier printed circuit board along with a TOSA, and all of the standardized components which will not vary from one transceiver to another can all be mounted together on the main transceiver printed circuit board. Removing the variable components from the main transceiver printed circuit board facilitates mass production of the main transceiver printed circuit boards by allowing the use of high speed automated techniques. Placing the variable biasing components on a separate carrier printed circuit board with the TOSA further allows easier testing of the output characteristics of the semiconductor laser within the TOSA and tailoring the biasing components to normalize the performance of the laser. In this way an efficient procedure may be established for testing the output characteristics of large numbers of semiconductor lasers and individually sizing and installing the biasing and compensating components. In a final step of the assembly process, the mass produced main transceiver printed circuit boards may be connected to any of the individually normalized TOSA assemblies, to complete the construction of an improved optoelectronic transceiver.